The mitral valve depends on adequate apposition or alignment between the anterior and posterior leaflets along a relatively long surface area under high pressure conditions. Typically, the contact surface is about 12 mm in a direction perpendicular to the anterior-posterior direction and this provides little margin of safety. The leaflet margins are attached to numerous fine chords suspended from attachment points along the inner surface of the left ventricle. Although these attachments are often referred to as papillary muscles, there is often a very diffuse arc-shaped attachment for each of the groups of chords to the endocardial surface. Unfortunately, this anchor point (i.e., the inner wall of the left ventricle) must move with each heartbeat and so the distance between the attachment of the leaflet edges is constantly changing. The chordal lengths may also change—typically increasing with age and degeneration and the chords frequently do not lengthen in a symmetrical fashion. This leads to variations in their lengths at all-important points of coaptation. Chords may also rupture. In addition, the mitral annulus changes diameter with each heartbeat such that its surface area changes by about 40% with each systole. As the heart enlarges, the annulus of the mitral valve can enlarge as well. In short, there are many variables affecting proper functioning of the mitral valve. The anatomy, such as the leaflet length, the chordal length and the annular length/diameter can change. The attachment points can change as the ventricle changes shape. More importantly, all of these aspects can change simultaneously. For example, a patient may have ischemic mitral regurgitation which pulls the posterolateral valve attachments away from their natural coaptation points and leads to an opening in this area of the mitral valve. This can be further affected if the chordal lengths are changed by even minor degrees of degenerative disease.
Mitral valve pathology has changed remarkably since the origin of open heart surgery one generation ago. Initially, the most common pathology or condition was rheumatic mitral valve disease. This produced thickened, impliable leaflets with grossly deformed chords, or chordae tendinae, often combined with fusion of the two leaflets. This valve was not suitable for any type of plastic procedure and, accordingly, numerous valve prostheses were developed to replace the entire valve, i.e., the annulus, leaflets and chords. Now, except in centers with high rates of immigration from third world countries, rheumatic mitral valve disease is a relatively uncommon indication for surgery. Various forms of degeneration ranging from gross billowing of leaflets to relatively minor chordal lengthening as well as ischemic mitral valve pathology are most commonly encountered. Recently, it has become apparent that combinations of these two problems are relatively common. In both of these situations, the mitral valve leaflets are soft, pliable and can be retained over the long-term in various repair procedures. Unfortunately, despite the fact that the leaflet tissue is suitable for retention, mitral repair is performed for less than half of the cases where mitral regurgitation is the problem. In surgical centers where mitral repair is not practiced, valves are often discarded and replaced.
One main problem is that mitral valve repair technology has not kept pace with the change in mitral valve pathology. Mitral valve repair is more an art than a science and requires a constant interaction between visual inspection and post operative results, as evidenced by transesophageal echocardiography (TEE). Few surgeons or surgical centers are equipped for or capable of performing this type of work on a routine basis. Many surgeons only perform mitral annuloplasty with rings that reduce the diameter of the annulus. These rings may appear to be a solution for a variety of problems but are not ideal for many ischemic and degenerative disease conditions.
Despite many attempts, the homograft mitral valve replacement is not an operation which can be performed reliably. It could have potential advantages in third world countries or in cases of infection. Failures occur because of the unreliability of attachment of the chords to the left ventricle. It is not difficult to anchor the valve in the annulus. However, it is virtually impossible to ensure that the chords are correctly spaced inside the ventricle to produce a competent valve. Again, the inner surface of the ventricle is a moving surface and it is almost impossible to guarantee that a chord extending from a leaflet edge will be fixed in such a way that the anterior and posterior leaflets are reliably aligned during valve operation.
Various other repair procedures are performed, but these are limited to the removal of leaflet tissue which is poorly supported and to chordal shortening and replacement. Many valves simply remain unrepaired due to the shortage of acceptable techniques and apparatus. The sophisticated procedures are acquired art forms that many surgeons either cannot master or do not have the time and opportunity to master.
Thirty years of valve surgery have indicated that the native leaflet tissue is the most reliable valve material. Despite numerous attempts to produce durable leaflet replacements, none have been found. The cost of demonstrating the value of a new material is extremely high. However, chordal replacement with polytetrofluorethylene is durable and highly satisfactory. Therefore, this at least provides a proven, reliable material to suspend leaflet tissue.
It is also clear that annuloplasty rings are durable, well-tolerated and do not require long-term anticoagulation. They fix the annular dimensions and reliably reduce one of the most important variables (i.e., the mitral annulus diameter) in mitral valve competence.
Regulatory issues in this field are the single most expensive factor. Next generation valve prosthesis designs are therefore most desirably based on the numerous available annuloplasty devices.
To properly and consistently repair the mitral valve, these variables must be fixed—the annular diameter, the leaflet length, the chordal length and the attachment point of the chords. Fortunately, the leaflet length is relatively constant. The annulus diameter can be fixed by the annuloplasty ring. The chords can be replaced by polytetrofluorethylene suture to fix their length. The missing variable is the attachment of the chords to the left ventricle. To date, this remains a troublesome variable to the valve repair.
Ischemic mitral regurgitation occurs when there is ventricular dysfunction which causes the posterolateral attachments of the mitral valve to be drawn away from the annulus in systole. This pulls the two leaflet edges apart at their point of coaptation and produces an asymmetrical regurgitant jet or, in other words, blood flow in the wrong direction through the valve. In its pure form, the leaflets, the chords and the attachment points are all anatomically normal. Sometimes there is a relative discrepancy between the distance the anterior leaflet is drawn inward relative to the posterior leaflet so they are not just separated from edge-to-edge but also there is a step deformity of the junction point. The patient may also have some underlying mild degree of degenerative deformity which may initially cause a mild, but well-tolerated degree of mitral regurgitation. However, the regurgitation often becomes severe after left ventricular ischemia occurs.
Some repair techniques apply tight annuloplasty rings which serve to buckle the leaflets and draw them together. This often leaves a degree of mitral regurgitation and mitral stenosis results. Annuloplasty can be accompanied by a modification of the Alfieri edge-to-edge repair, more recently referred to as the bowtie repair. With this technique, the surgeon merely sews the anterior leaflet to the posterior leaflet at the point of maximal distraction. This produces a two orifice valve with more stenosis.
Devices and methods are necessary that preserve the leaflet tissue but provide for virtually guaranteed coaptation of the leaflets by fixing some of the variables responsible for regurgitation. Other devices and methods are necessary that do not simply reduce the diameter of a heart valve annulus, but allow more specialized treatment tailored to patient needs.